1. Field of the Invention
The present invention relates to a photomask having small pitch images of openings for fabricating an opening of a semiconductor memory device, a photolithographic method using the same, and a semiconductor memory device fabricated using the same method.
2. Description of the Related Art
As semiconductor memory devices become more highly integrated, the line width of interconnections becomes reduced. Due to reduction of the line width of interconnections, the size of openings for electrically connecting conductive regions and interconnections, that is, the size and the pitch of contact holes or via holes, also becomes reduced. However, due to optical limitations, when ultraviolet light having a wavelength of 365 nm (i-line UV) is used as a light source, the resolution limit of a square opening is 0.35 μm, and 0.30 μm in the case of deep ultraviolet (DUV).
Thus, a photoresist flow process has been introduced as a method for forming small openings below the resolution limit. The photoresist flow process comprises the steps of: projecting the images of openings of a photomask on a photoresist layer; and reducing the size of the images of openings by applying thermal energy for a given time to reach a temperature above the glass transition temperature of the photoresist, and then flowing the photoresist.
However, in order to apply the photoresist flow process, a given quantity of the photoresist should exist between the images of openings. For example, in a case where the photoresist layer is formed to a thickness between about 0.8 μm and about 1.2 μm, the photoresist is flowed only when the distance between the images of openings is at least 0.25 μm. This means that the photoresist can be flowed only when a given quantity of photoresist exists around the images of openings.
However, in conventional high-integration devices, the openings are very small, the pitch between the openings is also small, and the quantity of photoresist required to apply the photoresist flow process does not exist around the openings, therefore, it is not possible for the photoresist flow process to be applied.